John Brennan denied at his Senate confirmation hearing as CIA director accusations that he played a central role in the agency's torture of suspected terrorists, and suggested he was misled as a CIA senior official over the value of information obtained through waterboarding.

Brennan faced lengthy questioning over the CIA's abduction and abuse of alleged terrorists at secret "black sites", following a confidential 6,000-page Senate report that Brennan described as "very concerning and disturbing" in its evidence that the agency misrepresented and lied about the value of "enhanced interrogation techniques".

Obama's nomination to be CIA director also made a forceful defence of the White House's drones policy, of which he has been a principal architect, saying that the public does not appreciate the "agony" the government goes through to avoid civilian deaths.

Brennan was pressed into a commitment to keep Congress informed on intelligence operations after lengthy battles with the White House over access to classified information, including the legal basis for the assassination of US citizens by drone.

Brennan handled the questions with confidence, in contrast with Chuck Hagel's disjointed appearance at his confirmation hearing for US defence secretary last week, and looked likely to win approval as CIA director.

Brennan was closely questioned about what he knew of the CIA's abduction and torture of suspected terrorists when he was the agency's executive deputy director after 9/11. It was put to him that another senior CIA official said Brennan helped "set the parameters" in devising enhanced interrogation techniques, known as EITs, and getting Justice Department authority for them.

Obama's nominee denied it, but acknowledged he knew about the programme and said he objected. Asked why he didn't intervene to stop it, he said that was not his job.

"I did not take steps to stop the CIA's use of those techniques. I was not in the chain of command of that programme," he said. "I was aware of the programme. I was CC'd in on some of those documents, but I had no oversight of it. I wasn't involved in its creation," he said.

"I had expressed my personal objections and views to some agency colleagues about certain of those EIT's, such as waterboarding, nudity and others where I professed my personal objections to it. But I did not try to stop it because it was something that was being done in a different part of the agency."

Brennan defended an interview with CBS in 2007 in which he said that IETs "saved lives" by gathering valuable intelligence.

"The reports I was getting subsequent to that and in the years after that, it was clearly my impression it was valuable information that was coming out," he said.

But he has since retreated from that view and told his confirmation hearing that the Senate's report on the CIA's detention and interrogation programme had disturbed him.

"There clearly were a number of things, many things, that I read in that report that were very concerning and disturbing to me. Ones that I would want to look into immediately if I were to be confirmed as CIA director. It talked about mismanagement of the programme, misrepresentation of information, providing inaccurate information, and it was rather damning in a lot of its language as far as the nature of these activities carried out," he said.

Although Brennan declined to call waterboarding torture, he pledged that under his direction the CIA will not again use such techniques.

Brennan also gave a repeated defence of the use of drones in the face of scepticism from some senators particularly over the legal authority for the president to order the killing of American citizens. Obama repeatedly refused to release the 50-page legal opinion giving him the power to sign off on the summary executions by drone, and only permitted members of the Senate intelligence committee to read it hours before the hearing after some threatened to hold up Brennan's confirmation.

Senator Ron Wyden pressed Brennan for more openness.

"It's the idea of giving any president unfettered power to kill an American without checks and balances that's so troubling. Every American has the right to know when their government believes it's allowed to kill them," he said.

Brennan said drones are only used as "a last resort to save lives" but agreed that "we need to optimise transparency while at the same time optimising secrecy" over the killings.

Brennan was also pressed over the CIA's refusal to reveal where it is carrying out drone strikes and promised to provide that information if he is confirmed as the agency's director.

Republican senator Susan Collins asked why the aim of the drone programme shifted from targeting al-Qaida's senior leadership to relatively junior figures in the organisation. Collins quoted the former US commander in Afghanistan, General Stanley McChrystal, as saying the drone strikes "are hated on a visceral level even by people who have never seen one" and asked Brennan if they may backfire by creating a new wave of terrorists.

Brennan said it is a concern.

"I think that is something that we have to be very mindful of, in terms of what the reaction is," he said. But he also claimed in parts of Yemen, where "people are being held hostage to al-Qaida", the drone strikes are welcomed.